But, actually, some same-sex birds do do it. So do beetles, sheep, fruit bats, dolphins, and orangutans. Zoologists are discovering that homosexual and bisexual activity is not unknown within the animal kingdom.

Roy and Silo, two male chinstrap penguins at New York's Central Park Zoo have been inseparable for six years now. They display classic pair-bonding behaviorentwining of necks, mutual preening, flipper flapping, and the rest. They also have sex, while ignoring potential female mates.

Wild birds exhibit similar behavior. There are male ostriches that only court their own gender, and pairs of male flamingos that mate, build nests, and even raise foster chicks.

Filmmakers recently went in search of homosexual wild animals as part of a National Geographic Ultimate Explorer documentary about the female's role in the mating game. (The film, Girl Power, will be screened in the U.S this Saturday at 8 p.m. ET, 5 p.m PT on MSNBC TV.)

The team caught female Japanese macaques engaged in intimate acts which, if observed in humans, would be in the X-rated category.

"The homosexual behavior that goes on is completely baffling and intriguing," says National Geographic Ultimate Explorer correspondent, Mireya Mayor. "You would have thought females that want to be mated, especially over their fertile period, would be seeking out males."

She argues that female macaques may enhance their social position through homosexual intimacy which in turn influences breeding success. Parish says, "Taking something that's nonreproductive, like mounting another femaleif it leads to control of a resource or acquisition of a resource or a good alliance partner, that could directly impact your reproductive success."